Bing Maps overlay stops working after awhile, requires refresh

[kring]

After a period of time, Cesium ion tokens expire and need to be refreshed. This mechanism appears to be working fine. After leaving leaving the Editor running for a couple of hours and then moving the camera to force some new tiles to load, I see this in the log:

LogCesium: Display: [2025-06-20 18:36:09.468] [info] [CesiumIonTilesetLoader.cpp:524] Refreshing Cesium ion token for url https://api.cesium.com/v1/assets/1/endpoint?access_token=[redacted].
LogCesium: Display: [2025-06-20 18:36:10.020] [info] [CesiumIonTilesetLoader.cpp:562] Successfuly refreshed Cesium ion token for url https://api.cesium.com/v1/assets/1/endpoint?access_token=[redacted].

And then Cesium World Terrain tiles continue to load as expected.

However, the Bing Maps Aerial overlay doesn't load. So we get scenes like this, with high res terrain and low res imagery:

[kring]

Possibly related to CesiumGS/cesium-native#749.

[kring]

Actually, this is much more serious than the Google case, because RefreshTileset doesn't fix it. It requires a restart of the process. Presumably the root cause here is our attempt to cache the Bing session in memory in order to avoid using extra sessions unnecessarily.